Lack of sleep:Insomnia can set off headaches. Once you have headaches, the pain can also make it harder to sleep at night. People with sleep disorders like obstructive sleep apnea are more likely to get headaches, in part because their sleep is disrupted.

Infections and allergies

Headaches are often a symptom of respiratory infections like a cold or the flu. Fever and blocked sinus passages can both set off headaches. Allergies trigger headaches via congestion in the sinuses, which causes pain and pressure behind the forehead and cheekbones.

Serious infections like encephalitis and meningitis cause more intense headaches. These illnesses also produce symptoms like seizures, high fever, and a stiff neck.

Medication overuse

Drugs that treat headaches can lead to more headaches if you use them more than two or three days a week. These headaches are known as medication overuse headaches, or rebound headaches. They occur almost every day, and the pain starts when you wake up in the morning.

Neurological causes

Nerve problems can sometimes be the source of head pain.

Occipital neuralgia: The occipital nerves run from the top of your spinal cord, up your neck, to the base of your skull. Irritation of these nerves can cause an intense, severe, stabbing pain in the back of your head or the base of your skull. The pain lasts from a few seconds to several minutes.

Giant cell arteritis: Also called temporal arteritis, this condition is caused by inflammation of blood vessels — including the temporal arteries along the side of the head. Symptoms can include headaches and pain in the jaw, shoulders, and hips, along with visual changes.

Trigeminal neuralgia: This condition affects the trigeminal nerve, which provides feeling to your face. It causes a severe and sudden jolt of shock-like pain in your face.

Other causes

Pain on the left side may also result from:

Tight headgear: Wearing a helmet or other protective headgear that’s too tight can put pressure on one or both sides of the head and cause pain.

Concussion: A hard hit to the head can cause this type of traumatic brain injury. Concussions produce symptoms like headaches, confusion, nausea, and vomiting.

Glaucoma: This rise in pressure inside the eye can lead to blindness. Along with eye pain and blurred vision, its symptoms can include a severe headache.

High blood pressure: Normally, high blood pressure doesn’t cause symptoms. But in some people headaches can be a sign.

Stroke: Blood clots can block blood vessels to the brain, cutting off blood flow and causing a stroke. Bleeding inside the brain can also cause a stroke. A sudden, severe headache is one warning sign of a stroke.

Brain tumor: A tumor can cause an intense, sudden headache along with other symptoms such as vision loss, speech problems, confusion, trouble walking, and seizures.

Migraine

Feels like: An intense, throbbing pain, often one side of the head. The pain is often accompanied by symptoms like nausea, vomiting, sound and light sensitivity, and auras.

Auras are changes in vision, speech, and other sensations. They occur before the migraine starts.

Symptoms include:

flashes of light, shapes, spots, or lines in your field of vision

numbness in your face or on one side of your body

vision loss

trouble speaking clearly

hearing sounds or music that isn’t there

Cluster

Cluster headaches are rare but intensely painful headaches. They get their name from their pattern. The headaches arrive in clusters over a period of days or weeks. These cluster attacks are followed by remissions — headache-free periods that can last for months or years.

Feels like: Intense pain on one side of your head. The eye on the affected side might be red and watery. Other symptoms include a stuffed or runny nose, sweating, and flushing of the face.

Chronic

Chronic headaches can be any type — including migraine or tension headaches. They’re called chronic because they happen at least 15 days a month for six months or more.

Feels like: A dull throbbing pain, intense pain on one side of the head, or a vice-like squeezing, depending on which type of headaches you get.

Make an appointment to see your doctor if you have new headaches or your headaches have become more severe. Your doctor may send you to a headache specialist called a neurologist.

Your doctor will do a physical exam. You’ll be asked about your medical history and what symptoms you’re having.

They might ask you questions like these:

When did the headaches start?

What does the pain feel like?

What other symptoms do you have?

How often do you get headaches?

What seems to trigger them?

What makes the headaches better? What makes them worse?

Is there a family history of headaches?

Your doctor may be able to diagnose your headache based on symptoms alone. But if they aren’t sure about what’s causing your headaches, they may recommend one of these imaging tests:

A CT scan uses a series of X-rays to create cross-sectional pictures of your brain. It can diagnose bleeding in your brain and certain other abnormalities.

A MRI uses powerful magnets and radio waves to create detailed pictures of your brain and its blood vessels. It provides a more detailed brain image than a CT scan. It can help diagnose strokes, bleeding in the brain, tumors, structural problems, and infections.